It has been recognized that due to environmental concerns and newly enacted rules and regulations, saleable petroleum products must meet lower and lower limits on contaminates, such as sulfur and nitrogen. New regulations require essentially complete removal of sulfur from liquid hydrocarbons that are used in transportation fuels, such as gasoline and diesel. For example, ultra low sulfur diesel (ULSD) requirements are typically less than about 10 ppm sulfur.
A mild hydrocracking unit, which often includes a hydrotreating zone and a hydrocracking zone, is one method to produce diesel boiling range hydrocarbons with a reduced level of sulfur. However, typical mild hydrocracking units generally cannot produce diesel meeting the ultra low sulfur requirements with acceptable cetane numbers. For example, product from a common mild hydrocracking unit still has about 100 to about 2000 ppm of sulfur and a relatively low cetane number of about 30 to about 40.
Attempts to improve the quality of the effluent from the mild hydrocracking unit are known, but do so at the expense of overtreating the higher boiling components or through additional high pressure vessels. Overtreated higher boiling components are generally not suitable for subsequent fluid catalytic cracking. Additional high pressure vessels require a large capital investment and are more costly to operate. Moreover, the hydrogen requirements for these additional high pressure vessels also require a costly recycle gas compressor, which also adds further capital investment and operating costs. For example, a typical high pressure vessel added to a mild hydrocracking unit typically requires a relatively large portion of the hydrogen recycle gas (up to about 10,000 SCF/B, for instance).
Other attempts to reduce the sulfur content of hydrocarbonaceous streams employ a two-phase reactor (i.e., liquid hydrocarbon stream and solid catalyst) with pre-saturation of hydrogen. See, e.g., Schmitz, C. et al., “Deep Desulfurization of Diesel Oil: Kinetic Studies and Process-Improvement by the Use of a Two-Phase Reactor with Pre-Saturator,” CHEM. ENG. SCI., 59:2821-2829 (2004). These two-phase systems only use enough hydrogen to saturate the liquid-phase in the reactor. As a result, the reactor systems of Schmidt et al. have the shortcoming that as the reaction proceeds and hydrogen is consumed, the reaction rate decreases due to the depletion of the dissolved hydrogen. As a result, such two-phase systems are limited in practical application and in maximum conversion rates.
Although a wide variety of process flow schemes, operating conditions and catalysts have been used in commercial petroleum hydrocarbon conversion processes, there is always a demand for new methods and flow schemes that provide more useful products and improved product characteristics. In many cases, even minor variations in process flows or operating conditions can have significant effects on both quality and product selection. There generally is a need to balance economic considerations, such as capital expenditures and operational utility costs, with the desired quality of the produced products.
There is a continuing need, therefore, for improved and cost effective methods to produce hydrocarbon streams that meet increasingly stringent product requirements. In particular, there is a need to provide ULSD in a cost effective and efficient manner without overtreating the heavier portions of the product streams.